hi steve, as far as I know DPX files do not contain an alpha. In the saver, there is an option to save the alpha to Color which will render a black and white matte, be sure to change the name of the file when you select "save Alpha to Color" otherwise it will overwrite what you've already rendered.

Although dpx spec does support an alpha channel it is in my experience very rarely used and a lot (i.e. most) of software packages can't deal with it.

The usual matte delivery pipeline for features is to provide a second set of dpx files that are 10 bit linear (as opposed to log) and use the RGB channels for the matte, with the same file numbering, resolution etc. Sometimes three different mattes are provided in the same file by using the Red Green and Blue files to carry the three mattes but for single mattes the same matte data is put into all three colour channels.

What does it matter whether it's efficient or not? It is part of the spec. Compression is in the spec as well.

The DPX format stores image data by pixels, separating each pixel into its component values (also called samples in other formats) and storing each value in a separate element (also called a color plane). Image data components may be stored in up to eight elements total.

For example, an 8-bit gray-scale pixel contains only one component and is therefore stored as one component value only in one element. A 32-bit RGBA pixel contains four components that may be stored across four elements as four 8-bit component values or may be stored in a single element as one 32-bit component value.

As you can see, the DPX format is quite flexible in allowing you to store your image data in any way that makes reading and writing the data the most efficient for your computer hardware.

All components must be the same size across all elements. Valid component sizes are 1-, 8-, 10-, 12-, and 16-bit integers and 32- and 64-bit reals (IEEE floating-point). All components must be stored as words using 32-bit boundaries.

Descriptor specifies the type of component stored by the element and its pixel-packing order. There are 256 possible values to this field; the following are defined:

It matters a lot. Transfer time, storage space, necessary network bandwidth, human time waiting for transfers and uploads. All of these things cost money. Sometimes a lot of money. It's one thing to transfer 8 MB frames when they're actual artwork. It's quite another when they're mattes that could be 10k with no loss of quality or utility.

Uh those things are not what matters in this case of PROPERLY SUPPORTING THE SPEC. Those are pipeline issues. DPX supports compression. Literally, the spec allows for an alpha in any bit depth from 8 bit to 64bits (yeah dpx supports 64 bits per channel crazy world) . Just like EXR the channels can all be different bit depths but you only get 8 of them in DPX.

So fusion should support at least RGBA in all bit depth of DPX because that is the spec.

It matters a lot. Transfer time, storage space, necessary network bandwidth, human time waiting for transfers and uploads. All of these things cost money. Sometimes a lot of money. It's one thing to transfer 8 MB frames when they're actual artwork. It's quite another when they're mattes that could be 10k with no loss of quality or utility.